Light-emitting diodes (LEDs), when disposed on a circuit, accept electrical impulses from the circuit and convert the impulses into light signals. LEDs are typically energy efficient and can have a long lifetime.
Various types of LED exists, including air gap LEDs, GaAs light-emitting diodes, polymer LEDs, and semi-conductor LEDs, among others. Although most LEDs in current use are red, LEDs may take any color. In addition, when several LEDs, each of a commonly used primary color—red, blue or green—are combined in different proportions, the combination can generate almost any color in the visible spectrum in response to changing electrical signals. Alternatively, a single LED may be designed to include dies, each with a primary color (i.e., red, blue, and green), which can be combined to generate almost any colors.
Traditionally, LEDs have been poor in their ability to generate sufficient light output for illumination. Accordingly, LEDs have been used in low light environments, such as a flashing light on a modem or other computer components, or as a digital display of a wristwatch. However, over the past several years, the ability of the LEDs to generate sufficiently high intensity light output has increased substantially. Thus, LEDs have recently been used in arrays for flashlights, traffic lights, scoreboards and similar displays, and as television displays.
Despite this new development, the manufacturing of high-intensity LEDs is still a challenging process. In particular, it has been difficult to precisely predict the performance of an LED-based product in terms of total light output and quality of the output. Specifically, as an LED increases in output intensity, the quality of the output decreases.
In general, during manufacturing, the LEDs are tested for total light output and subsequently classified into “bins”. Depending on the manufacturer, however, this classification can happen either after the semiconductor wafer has been sliced into individual dies or after the die is packaged into the LED plastic housing. In either case, for each LED, a light output determination is made of both wavelength (the color of light) and intensity of the output. The LEDs are then sorted and sold based on the bin-type for which they have been classified.
It should be noted that even with bin sorting, there remain substantial and perceptible differences in the light output amongst LEDs. This difference, as a result, can lead to perceptible differences in light output between otherwise identical LED-based products. Moreover, if “additive mixing” is employed, wherein a few LEDs of different colors are mixed to produce other colors, should the color in one or more of these LEDs be off, then the results of the additive mixture will also be off.
Furthermore, the light output from the LEDs may change over time as a result of a variety of factors, and can also contribute to perceptible differences in light output. For instance, the LEDs may degrade or shift in color output over time, as part of a normal deterioration of the LEDs. In addition, long term exposure of the LEDs to high heat, or requiring the LEDs to maintain high intensity light output over an extended period of time, such that the LEDs produce too much heat, or if the heat can not be removed sufficiently quickly away from the LEDs, such effects can accelerate the deterioration of the LEDs and lead to permanent changes in the LEDs.
Accordingly, it is desirable to provide a system which can calibrate and adjust the light output of each LED, so that uniformity in light output by the LED can be achieved, during manufacturing, and subsequently maintained during the lifetime of the LED, either alone or in combination.